Bump third_party/imgui/repo from `6228c2e` to `763100b` #88387

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 8m 13s
Build on Linux (coverage)	10m 26s
Build on Linux (fake, gcc_release, clang, simulated)	1h 27m 5s
Build on Linux (gcc_debug)	48m 34s
Build on Linux (python_lib)	18m 58s
Build on Linux (python lighting-app)	6m 0s
	3h 59m 16s

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